Flaws of Cost-Benefit Analysis
Cost-Benefit Analysis struggles as an approach where a project has cash flows that come in over a number of periods of time, particularly where returns vary from period to period. In these cases, use Net Present Value (NPV) and Internal Rate of Return (IRR) calculations together to evaluate the project, rather than using Cost-Benefit Analysis. (These also have the advantage of bringing "time value of money" into the calculation.)
Also, the revenue that will be generated by a project can be very hard to predict, and the value that people place on intangible benefits can be very subjective. This can often make the assessment of possible revenues unreliable (this is a flaw in many approaches to financial evaluation). So, how realistic and objective are the benefit values used?